When you install Microsoft Office 2010 on a workstation running Office 2007 (or Office 2003 or Office XP), there is now an option to have both those versions of Office coexist on that workstation and within the same Windows profile. In a manual installation, after selecting the Customize installation option (after accepting the EULA), you can then select individual applications to remove in the Upgrade tab as shown in
Figure 1.

Outlook 2010 (and SharePoint Workspace 2010 formerly known as Groove) is the only Office application that can’t coexist with previous versions. This situation creates a possible conflict between the Upgrade tab and the Installation Options tab in Figures 1 and 2. If you tell Office to not replace Outlook in the Upgrade tab, but also tell it to install Outlook 2010 in the Installation Options tab, Office will replace your existing Outlook version and install Outlook 2010. That is the default. And it will not ask if you're sure you want to do this.

There might be circumstances where you need to retain Outlook 2007 but you're ready to move on to Office 2010. Perhaps applications or add-ons you're using with Outlook 2007 still need to be tested or developed for Outlook 2010. I recommend upgrading all applications at once, but different companies might have unique requirements for Office. Outlook retains its settings and Outlook profiles when an upgrade to Outlook 2010 is done.

During the beta phase of Office 2010, you had to manually choose a unique destination folder for the binaries to prevent Office from overwriting your current installation. This is now done automatically when you tell Office 2010 RTM not to remove a previous version. For example, with Office 2010 installed while retaining Office 2007, the applications by default are saved in separate locations as follows:

Office 2007: %ProgramFiles%\Microsoft Office\Office12

Office 2010: %ProgramFiles%\Microsoft Office\Office14

After Office 2010 is installed, links to applications for both versions reside in the Start menu; however, as Figure 3 shows, there is only one shortcut for Outlook. (From Figure 3, you can also see that Microsoft has dropped “Office” from the application names as well.)